Evaporative fuel-processing system for internal combustion engines

ABSTRACT

An evaporative fuel processing system adapted to be capable of detecting abnormality of an evaporative emission control system for storing, in a canister, evaporative fuel from a fuel tank for holding fuel to be supplied to an internal combustion engine, and purging evaporative fuel into the intake system of the engine. A first control valve is arranged across a passage extending between the fuel tank and the canister. A second control valve is arranged across a passage extending between the canister and the intake system of the engine. A third control valve is provided for an air inlet part of the canister communicatable with the atmosphere. Through operating these control valves to open and close them, the evaporative emission control system is negatively pressurized, and abnormality of this system is detected based on the pressure detected in this negatively pressurized state thereof. Timing for carrying out abnormality determination is determined depending on conditions of the fuel tank. Before starting the whole process for abnormality diagnosis of the system evaporative fuel stored in the canister is allowed to be purged for a predetermined time period. When the temperature of fuel in the fuel tank exceeds a predetermined value, the abnormality determination is inhibited.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an evaporative fuel-processing system forinternal combustion engines, and more particularly to an evaporativefuel-processing system for internal combustion engines, which is capableof performing abnormality diagnosis of an evaporative emission controlsystem for purging evaporative fuel generated from a fuel tank of theengine into an intake system of same.

2. Prior Art

Conventionally, there has been widely used an evaporativefuel-processing system for internal combustion engines, which comprisesa fuel tank, a canister having an air inlet port provided therein, afirst control valve arranged across an evaporative fuel-guiding passageextending from the fuel tank to the canister, and a second control valvearranged across a purging passage extending from the canister to theintake system of the engine.

A system of this kind temporarily stores evaporative fuel in thecanister, which is then purged into the intake system of the engine.

Whether a system of this kind is normally operating can be checked, forexample, by comparing a first value of an air-fuel ratio correctioncoefficient assumed when purging of evaporative fuel into the intakesystem is stopped and a second value of the air-fuel ratio correctioncoefficient assumed when purging of evaporative fuel is effected, aftercompletion of warming-up of the engine. That is, when the evaporativefuel-processing system is normally functioning to purge evaporative fuelinto the intake system, an air-fuel mixture supplied to the engine isenriched by the evaporative fuel purged. The enriched air-fuel mixtureis detected by an air-fuel ratio sensor, e.g. an O₂ sensor, and hencethe air-fuel ratio correction coefficient calculated for feedbackcontrol of the air-fuel ratio assumes a smaller value. Therefore,monitoring of the manner of decrease in the air-fuel ratio correctioncoefficient enables to determine abnormality of the evaporativefuel-processing system. This abnormality diagnosis method is disclosedin U.S. Pat. No. 5,085,194.

However, the above abnormality diagnosis method using the air-fuel ratiocorrection coefficient suffers from a problem that in the case where aleak of evaporative fuel occurs from defective seals provided at pipingconnections, valves, the fuel tank, etc. of the system, (e.g. a seal ata filler cap of the fuel tank), it is impossible to detect the leak bythe above method, which can result in emission of a large amount ofevaporative fuel into the air.

SUMMARY OF THE INVENTION

It is the object of the invention to provide an evaporativefuel-processing system for an internal combustion engine, which iscapable of detecting abnormality of an evaporative emission controlsystem, by detecting whether there occurs a leak of evaporative fuelfrom seals provided at piping connections, etc. of the system.

To attain the above object, according to a first aspect of theinvention, there is provided an evaporative fuel-processing system foran internal combustion engine having an intake system including anevaporative emission control system, having a fuel tank a canistercontaining an adsorbent, the canister having an air inlet portcommunicatable with the atmosphere, an evaporative fuel-guiding passageextending between the canister and the fuel tank, a first control valvearranged across the evaporative fuel-guiding passage, an evaporativefuel-purging passage extending between the canister and the intakesystem, and a second control valve arranged across the evaporativefuel-purging passage.

The evaporative fuel-processing system according to the first aspect ofthe invention is characterized by having an abnormality-determiningsystem which comprises:

tank internal pressure-detecting means for detecting pressure within thefuel tank;

negatively-pressurizing means for negatively pressurizing theevaporative emission control system; and

abnormality-determining means for determining abnormality of theevaporative emission control system based on the pressure within thefuel tank detected after the evaporative emission control system hasbeen negatively pressurized by the negatively-pressuring means.

Preferably, the abnormality-determining means determines the abnormalityof the evaporative emission control system based on a rate of change inthe pressure within the fuel tank occurring before the evaporativeemission control system is set to a predetermined negatively-pressurizedcondition by the negatively-pressurizing means and a rate of change inthe pressure within the fuel tank occurring after the predeterminednegatively-pressurized condition of the evaporative emission controlsystem has been established.

Preferably, the evaporative fuel-processing system includes tankcondition-detecting means for detecting conditions of the fuel tank,wherein the abnormality-determining means carries out abnormalitydetermination when a predetermined time period has elapsed after theevaporative emission control system was negatively pressurized thepredetermined time period being corrected by a correcting time periodset in response to the conditions of the fuel tank detected by the tankcondition-detecting means.

Preferably, the abnormality-determining means determines abnormality ofthe evaporative emission control system by comparing a value of aparameter indicative a rate of change in the pressure within the fueltank detected after the evaporative emission control system has beennegatively pressurized by the negatively-pressurizing means with apredetermined reference value, the predetermined reference value beingdetermined according to a time period required for setting theevaporative emission control system to the predeterminednegatively-pressurized condition by the negatively-pressurizing means.

Preferably, the evaporative fuel-processing system includes means forpurging evaporative fuel stored in the canister for a predetermined timeperiod before the abnormality-determining process is started by theabnormality-determining system.

Preferably, the evaporative fuel-processing system includes fueltemperature-detecting means for detecting the temperature of fuelcontained in the fuel tank, and determination-inhibiting means forinhibiting execution of abnormality-determining process by theabnormality-determining system when the fuel temperature detectedexceeds a predetermined value.

According to a second aspect of the invention, the evaporativefuel-processing system is characterized by having anabnormality-determining system which comprises:

engine operating condition-detecting means for detecting operatingconditions of the engine;

a third control valve for effecting and cutting off the communication ofthe air inlet port of the canister with the atmosphere;

tank internal pressure-detecting means for detecting pressure within thefuel tank;

negatively-pressurizing means for setting the evaporative emissioncontrol system to a predetermined negatively-pressurized condition bycontrolling the first to third control valves when it is detected by thethe engine operating condition-detecting means that the engine is inoperation;

a first rate of change-detecting means for detecting a rate of change inthe pressure within the fuel tank caused by controlling opening andclosing of the first control valve;

a second rate of change-detecting means for detecting a rate of changein the pressure within the fuel tank caused by closing the secondcontrol valve after the negatively-pressurized condition of theevaporative emission control system has been established; and

abnormality-determining means for determining abnormality of theevaporative emission control system based on results of detection by thefirst and second rate of change-detecting means.

Preferably, the evaporative fuel-processing system of the second aspectof the invention also includes tank condition-detecting means fordetecting conditions of the fuel tank, wherein theabnormality-determining means carries out abnormality determination whena predetermined time period has elapsed after the evaporative emissioncontrol system was negatively pressurized, the predetermined time periodbeing corrected by a correcting time period set in response to theconditions of the fuel tank detected by the tank condition-detectingmeans.

Preferably, also in the evaporative fuel-processing system of the secondaspect of the invention, the abnormality-determining means determinesabnormality of the evaporative emission control system by comparing avalue of a parameter indicative of a rate of change in the pressurewithin the the fuel tank detected after the evaporative emission controlsystem has been negatively pressurized by the negatively-pressurizingmeans. With a predetermined reference value during the negativelypressurizing, the predetermined reference value being determinedaccording to a time period required for setting the evaporative emissioncontrol system to the predetermined negatively-pressurized condition bythe negatively-pressurizing means.

Preferably, the abnormality-determining system includes fuelamount-detecting means for detecting an amount of fuel contained in thefuel tank, the abnormality-determining means determines the abnormalityof the evaporative emission control system based on results of detectionby the first and second rate of change-detecting means and the fuelamount-detecting means.

Preferably, the evaporative fuel-processing system according to thesecond aspect of the invention also includes means for purgingevaporative fuel stored in the canister for a predetermined time periodbefore the abnormality-determining process is started by theabnormality-determining system.

Preferably, the evaporative fuel-processing system according to thesecond aspect of the invention also includes fuel temperature-detectingmeans for detecting the temperature of fuel contained in the fuel tank,and determination-inhibiting means for inhibiting execution ofabnormality-determining process by the abnormality-determining systemwhen the fuel temperature detected exceeds a predetermined value.

According to a third aspect of the invention, the evaporativefuel-processing system is characterized by having anabnormality-determining system which comprises:

engine operating condition-detecting means for detecting operatingconditions of the engine;

a third control valve for effecting and cutting off the communication ofthe air inlet port of the canister with the atmosphere;

tank internal pressure-detecting means for detecting pressure within thefuel tank;

negatively-pressurizing means for setting the evaporative emissioncontrol system to a predetermined negatively-pressurized condition bycontrolling the first to third control valves when it is detected by thethe engine operating condition-detecting means that the engine is inoperation; and

abnormality-determining means for effecting a determination as towhether or not the evaporative emission control system is abnormallyfunctioning, when a predetermined time period has elapsed during thenegatively-pressurizing process by the negatively-pressurizing means.

Preferably, the abnormality-determining system includes evaporative fuelgeneration rate-detecting means for detecting a parameter of an amountof evaporative fuel generated per unit time within the fuel tank, theabnormality-determining means determining that the evaporative emissioncontrol system is abnormal on condition that the parameter indicative ofthe amount of evaporative fuel generated per unit time within the fueltank is smaller than a predetermined value.

The above and other objects, features, and advantages of the inventionwill become more apparent from the ensuing detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the whole arrangement of aninternal combustion engine and an evaporative fuel-processing systemtherefor according to an embodiment of the invention;

FIG. 2 is a graph showing test data obtained when there occurs no leakof evaporative fuel from the system;

FIG. 3 is a graph showing test data obtained when there occurs a leak ofevaporative fuel from the system;

FIG. 4 is a timing chart showing operation of first and secondelectromagnetic valves, a drain shut valve, and a second control valve,and changes in pressure within a fuel tank (tank internal pressure), allappearing in FIG. 1;

FIG. 5 is a flowchart of a routine for determining whether monitoringconditions are satisfied;

FIG. 6 is a flowchart of a program for carrying out abnormalitydiagnosis of an evaporative emission control system in FIG. 1;

FIG. 7 shows a table for calculating a parameter (fueltemperature-dependent correcting time period ΔTTF) used for theabnormality diagnosis;

FIG. 8 shows a table for calculating a parameter (fuel amount-dependentcorrecting time period ΔTVF) used for the abnormality diagnosis;

FIG. 9 shows a table for calculating a parameter (tank internalpressure-dependent correcting time period ΔTPTO) used for theabnormality diagnosis;

FIG. 10 shows a table for calculating a parameter(negatively-pressurizing time period-dependent correcting time periodΔTtmPT) used for the abnormality diagnosis;

FIG. 11 is a flowchart of an abnormality-determining routine carried outby the program of FIG. 6,

FIG. 12 is a flowchart of another abnormality-determining routinecarried out by the program of FIG. 6;

FIG. 13 is a timing chart showing operation of first and secondelectromagnetic valves, a drain shut valve, and a second control valve,and changes in the tank internal pressure;

FIG. 14 is a flowchart showing a manner of carrying out an abnormalitydiagnosis of the evaporative emission control system;

FIG. 15 is a flowchart of a routine for determining whether monitoringconditions are satisfied;

FIG. 16 is a flowchart of a routine for checking tank internal pressurewhen the interior of the fuel tank is open to the air;

FIG. 17 is a flowchart of a routine for checking changes in the tankinternal pressure;

FIG. 18 is a flowchart of a routine for reducing the tank internalpressure;

FIG. 19 is a flowchart of a leak down check routine for checking achange rate in the tank internal pressure when the evaporative emissioncontrol system is isolated from the intake pipe;

FIG. 20 is a flowchart of a routine for determining conditions of thesystem;

FIG. 21 is a flowchart of a routine for determining occurrence of anabnormality;

FIG. 22 shows a map used by the routine of FIG. 20 for determiningabnormality;

FIG. 23 is a flowchart of another example of the routine for determiningoccurrence of abnormality,

FIG. 24 (I), (II), and (III) show maps used by the routine of FIG. 23for determining abnormality;

FIG. 25 is a flowchart showing a manner of setting the valves for normalpurging;

FIGS. 26a and b are useful in explaining the influence of fueltemperature on the abnormality diagnosis; and

FIG. 27 is a schematic diagram showing the whole arrangement of aninternal combustion engine and an evaporative fuel-processing systemtherefor according to another embodiment of the invention.

DETAILED DESCRIPTION

The invention will now be described in detail with reference to thedrawings showing embodiments thereof.

Referring first to FIG. 1, there is illustrated the whole arrangement ofan internal combustion engine and an evaporative fuel-processing systemtherefor according to an embodiment of the invention.

In the figure, reference numeral 1 designates an internal combustionengine hereinafter simply referred to as “the engine”) having fourcylinders, not shown, for instance. Connected to the cylinder block ofthe engine I is an intake pipe 2 across which is arranged a throttlebody 3 accommodating a throttle valve 3′ therein. A throttle valveopening (θTH) sensor 4 is connected to the throttle valve 3′ forgenerating an electric signal indicative of the sensed throttle valveopening and supplying same to an electronic control unit (hereinafterreferred to as “the ECU”) 5.

Fuel injection valves 6, only one of which is shown, are inserted intothe interior of the intake pipe 2 at locations intermediate between thecylinder block of the engine I and the throttle valve 3′ and slightlyupstream of respective intake valves not shown. The fuel injectionvalves 6 are connected to a fuel pump 8 via a fuel supply pipe 7, andelectrically connected to the ECU 5 to have their valve opening periodscontrolled by signals therefrom.

A negative pressure communication passage 9 and a purging passage 10open into the intake pipe at respective locations downstream of thethrottle valve 3′, both of which are connected to an evaporativeemission control system 11, referred to hereinafter.

Further an intake pipe absolute pressure (PBA) sensor 13 is provided incommunication with the inferior of the intake pipe 2 via a conduit 12opening into the intake passage 2 at a location downstream of an end ofthe purging passage 10 opening into the intake pipe 2 for supplying anelectric signal indicative of the sensed absolute pressure within theintake pipe 2 to the ECU 5.

An intake air temperature (TA) sensor 14 is inserted into the intakepipe 2 at a location downstream of the conduit 12 for supplying anelectric signal indicative of the sensed intake air temperature TA tothe ECU 5.

An engine coolant temperature (TW) sensor 15 formed of a thermistor orthe like is inserted into a coolant passage filled with a coolant andformed in the cylinder block, for supplying an electric signalindicative of the sensed engine coolant temperature TW to the ECU 5.

An engine rotational speed (NE) sensor 16 is arranged in facing relationto a camshaft or a crankshaft of the engine 1, neither of which isshown. The engine rotational speed sensor 16 generates a pulse as a TDCsignal pulse at each of predetermined crank angles whenever thecrankshaft rotates through 180 degrees, the pulse being supplied to theECU 5.

A transmission 17 is interposed between driving wheels, not shown, andthe engine 1, such that the driving wheels are driven by the engine 1via the transmission 17.

A vehicle speed (VSP) sensor 18 is provided at the wheels for supplyingan electric signal indicative of the sensed vehicle speed (VSP) to theECU 5.

An oxygen concentration sensor (hereinafter referred to as “the O₂sensor”) 20 is mounted in an exhaust pipe 19 connected to the cylinderblock of the engine 1, for sensing the concentration of oxygen presentin exhaust gases emitted from the engine 1 and supplying an electricsignal indicative of the sensed oxygen concentration to the ECU 5.

An ignition switch (IGSW) sensor 21 detects an ON (or closed) state ofthe ignition switch IGSW, to detect that the engine 1 is in operation,and supplies an electric signal indicative of the ON state of theignition switch IGSW to the ECU 5.

The evaporative emission control system 11 is comprised of a fuel tank23 having a filler cap 22 which is removed for refueling, a canistercontaining activated carbon 24 as an adsorbent and having an air inletport 25 provided in an upper wall thereof, an evaporative fuel-guidingpassage 27 connecting between 5 the canister 26 and the fuel tank 23,and a first control valve 28 arranged across the evaporativefuel-guiding passage 27.

The fuel tank 23 is connected to fuel injection valves 6 via the fuelpump 8 and the fuel supply pipe 7, and has tank internal pressure (PT)sensor (hereinafter referred to as “the PT sensor”) 29 and a fuel amount(FV) sensor 30 (hereinafter referred to as “the FV sensor”) both mountedat an upper wall thereof, and a fuel temperature (TF) sensor(hereinafter referred to as “the TF sensor”) 31 penetrated through aside wall thereof. The PT sensor 29, FV sensor 30, and TF sensor 31 areelectrically connected to the ECU 5. The PT sensor 29 senses thepressure (tank internal pressure PT) within the fuel tank 23 andsupplies an electric signal indicative of the sensed tank internalpressure PT to the ECU 5. The FV sensor 30 senses an amount (FV) of fuelwithin the fuel tank 23 and supplies an electric signal indicative ofthe sensed fuel amount FV to the ECU . The TF sensor 31 senses the fueltemperature (TF) and supplies an electric signal indicative of thesensed fuel temperature TF to the ECU 5.

The first control valve 28 comprises a two-way valve 34 formed of apositive pressure valve 32 and a negative pressure valve 33, and a firstelectromagnetic valve 35 formed in one body with the two-way valve 34.More specifically, the first electromagnetic valve 35 has a rod 35 a afront end of which is fixed to a diaphragm 32 a of the positive pressurevalve 32. Further, the first electromagnetic valve 35 is electricallyconnected to the ECU 5 to have its operation controlled by a signalsupplied from the ECU 5. When the first electromagnetic valve 35 isenergized, the positive pressure valve 32 of the two-way valve 34 isforcedly opened to open the first control valve 28, whereas when thefirst electromagnetic valve 35 is deenergized, the valving(opening/closing) operation of the first control valve 28 is controlledby the two-way valve 34 alone.

A purge control valve 36 (second control valve) is arranged across thepurging passage 10, which has a solenoid, not shown, electricallyconnected to the ECU 5. The purge control valve 36 is controlled by asignal supplied from the ECU 5 to linearly change the opening thereof.That is, the ECU 5 supplies a desired amount of control current to thepurge control valve 36 to control the opening thereof.

A hot-wire type flowmeter (mass flowmeter) 37 is mounted across thepurging passage 10 at a location between the canister 26 and the purgecontrol valve 36. The hot-wire type flowmeter 37 utilizes the nature ofa platinum wire that when the platinum wire is heated by electriccurrent applied thereto and at the same time exposed to a flow of gas,the platinum wire looses its heat to decrease in temperature so that itselectric resistance decreases The output characteristic of the flowmeter37 varies according to the concentration and flow rate of evaporativefuel, and a purging flow rate of a mixture of evaporative fuel and air,and the flowmeter 37 generates and supplies an output signal accordingto the varying output characteristic thereof, to the ECU 5.

A drain shut valve 38 is mounted across the negative pressurecommunication passage 9 connecting between the air inlet port 25 of thecanister 26 and the intake pipe 2, and a second electromagnetic valve 39is mounted across the negative pressure communication passage 9 at alocation downstream of the drain shut valve 38, the drain shut valve 38and the second electromagnetic valve 39 constituting a third controlvalve 40.

The drain shut valve 38 has an air chamber 42 and a negative pressurechamber 43 defined by a diaphragm 41. Further, the air chamber 42 isformed of a first chamber 44 accommodating a valve element 44 a, asecond chamber 45 formed with an air-introducing port 45 a, and anarrowed communicating passage 47 connecting the second chamber 45 withthe first chamber 44. The valve element 44 a is connected via a rod 48to the diaphragm 41. The negative pressure chamber 43 communicates withthe second electromagnetic valve 39 via the communication passage 9, andhas a spring 49 arranged therein for resiliently urging the diaphragm 41and hence the valve element 44 a in the direction indicated by an arrowA.

The second electromagnetic valve 39 is constructed such that when asolenoid thereof is deenergized, a valve element 39 a thereof is in aseated position to allow air to be introduced into the negative pressurechamber 43 via an air inlet port 50 and an opening 39 b, and when thesolenoid is energized, the valve element 39 a is in a lifted position toclose the opening 39 b so that the negative pressure chamber 43communicates with the intake pipe 2 via the communication passage 9. Inaddition, reference numeral 51 indicates a check valve.

The ECU 5 comprises an input circuit having the functions of shaping thewaveforms of input signals from various sensors, shifting the voltagelevels of sensor output signals to a predetermined level, convertinganalog signals from analog-output sensors to digital signals, and soforth, a central processing unit (hereinafter called “the CPU”), memorymeans storing programs executed by the CPU and for storing results ofcalculations therefrom, etc., and an output circuit which outputsdriving signals to the fuel injection valves 6, the first and secondelectromagnetic valves 35, 39, and the purge control valve 36.

The outline of the manner of detecting abnormality of the evaporativeemission control system 11 in the evaporative fuel-processing systemconstructed as above will be described with reference to FIGS. 2 and 3.FIGS. 2 and 3 show changes in the pressure within the evaporativeemission control system 11 which will occur as time elapses afternegative pressure has been built within the system 11. FIG. 2 shows suchchanges in a case where no evaporative fuel leaks from the evaporativeemission control system 11, while FIG. 3 shows such changes in a casewhere there occurs a leak of evaporative fuel from the system 11.Further, the symbol of a indicates a curve obtained when the fuel tank23 is filled with the maximum amount of fuel, while the symbols b and cindicate curves obtained when the fuel tank contains ⅓ and ½ of themaximum amount, respectively.

As is clear from FIG. 2, when the evaporative emission control system 11is held in a negatively-pressurized state, the pressure within thesystem 11 progressively increases toward the atmospheric pressure at aslow rate due to an insignificant or inevitably permitted amount of leakfrom seals of the valves, etc., even if the seals have good performance.However, as shown in FIG. 3, the rate of increase in the pressure withinthe system 11 in this case (and hence the rate of leak of evaporativefuel in a normal purging mode) increases when the sealing of pipingconnections, etc. of the system 11 is faulty. Since the pressure withinthe system 11 can be detected by the PT sensor 29, it is possible todetermine abnormality of the system 11 based on the output from the PTsensor 29 outputted when the system is in the negatively-pressurizedstate.

FIG. 4 shows an example of changeover of operative states of the firstand second electromagnetic valves 35, 39, the drain shut valve 38, andthe second control valve 36 of the system, and changes in the tankinternal pressure PT resulting therefrom.

Specifically, the first electromagnetic valve 35 and the secondelectromagnetic valve 39 are both deenergized, when the engine is undera normal operating condition (i.e. in a normal purging mode), asindicated by (i) in the figure. When the IGSW sensor 21 detects the ON(or closed) state of the ignition switch IGSW, i.e. the engine is inoperation, the second control valve 36 is turned on or opened. In thisstate, the first control 15 valve 28 is controlled by the two-way valve34. More specifically, when the tank internal pressure PT exceeds apreset value of the positive pressure valve 32 of the two-way valve 34,the positive pressure valve 32 opens to allow evaporative fuel generatedfrom the fuel tank 23 to flow via the evaporative fuel-guiding passage27 into the canister 26, where it is temporarily adsorbed by theadsorbent 24. As mentioned above, the second electromagnetic valve 39 isin the deenergized (OFF) state under the normal operating condition(i.e. in the normal purging mode), and hence the drain shut valve 38 isopen, so that the outside air is supplied via the air-introducing port45 a to the canister 26, whereby evaporative fuel flowing into thecanister is purged together with the outside air thus introduced, viathe second control valve 36 through the purging passage 10.

When the fuel tank 23 is cooled by the outside air, etc., to increasethe negative pressure within the tank 23, i.e. reduce the absolutepressure within the fuel tank 23, the negative pressure valve 33 of thetwo-way valve 34 is opened to allow evaporative fuel stored in thecanister to return to the fuel tank 23.

When the engine 1 satisfies predetermined monitoring conditions,specified below, the first and second electromagnetic valves 35, 39, andthe purge control valve 36 are operated in a manner described below tocarry out an abnormality diagnosis of the evaporative emission controlsystem 11.

First the tank internal pressure PT is relieved to the atmosphere, overa time period indicated by (ii) in FIG. 4. That is, the firstelectromagnetic valve 35 is turned on or energized to force open thefirst control valve 28, and at the same time the second electromagneticvalve 39 is held in the OFF state to keep the drain shut valve 38 open,further with the second control valve 36 being held in the energized(ON) state, to thereby relieve the tank internal pressure PT to theatmosphere.

Then, the pressure within the evaporative emission control system 11 isdecreased, over a time period indicated by (iii) in FIG. 4. Morespecifically, while the first electromagnetic valve 35 and the secondcontrol valve 36 are held energized (ON), the second electromagneticvalve 39 is turned on, whereby the drain shut valve 38 is closed by apulling force acting on the diaphragm 41 created by negative pressurewithin the negative pressure communication passage 9 communicating withthe intake pipe 2. In this state, the evaporative emission controlsystem 11 is negatively pressurized by a gas-drawing force created bynegative pressure within the purging passage 10 communicating with theintake pipe 2.

Then, the leak down check is performed, over a time period indicated by(iv) in FIG. 4.

More specifically, the second control valve 36 is closed while thenegative pressurized state established over the preceding time period 3is maintained, followed by monitoring changes in the tank internalpressure PT by means of the PT sensor 29. If the sealing of theevaporative emission control system 11 is good, and hence there occursno significant leakage of evaporative fuel from the system 11 when theengine is under the aforementioned normal operating condition, i.e. thenormal purging mode, there hardly occurs a change in the tank internalpressure PT, as indicated by the two-dot chain line, whereas if thesealing of same is faulty, and hence there occurs a significant leak ofevaporative fuel from the system 11 when the engine is under the normaloperating condition or the normal purging mode, the tank internalpressure PT changes at a much larger rate than in the former case asindicated by the solid line, which enables to determine that theevaporative emission control system 11 is in an abnormal condition.

Next, there will be described in detail a manner of carrying out anabnormality diagnosis of the evaporative emission control system 11.

FIG. 5 shows a routine for determining whether the monitoring conditionsare satisfied, which permit to carry out monitoring of the evaporativeemission control system 11 with respect to leakage of evaporative fuel.The routine is executed as background processing.

First, at a step S1, it is determined whether or not the coolanttemperature TW detected by the TW sensor 15 falls between apredetermined lower limit value TWL (e.g. 70° C.) and a predeterminedhigher limit value TWH (e.g. 90° C.). If the answer to this question isaffirmative (YES), it is determined at a step S2 whether or not theintake air temperature TA detected by the TA sensor 14 falls between apredetermined lower limit value (e.g. 50° C.) and a predetermined higherlimit value (e.g. 90° C.). If the answer to this question is affirmative(YES), it is judged that the warming-up of the engine 1 has beencompleted, and then the program proceeds to a step S3.

At the step S3, it is determined whether or not the engine rotationalspeed NE detected by the NE sensor 16 falls between a predeterminedlower limit value NEL (e.g. 2000 rpm) and a predetermined higher limitvalue NEH (e.g. 4000 rpm). If the answer to this question is affirmative(YES), it is determined at a step S4 whether or not the intake pipeabsolute pressure PBA detected by the PBA sensor 13 falls between apredetermined lower limit value PBAL (e.g. 350 mmHg) and a predeterminedhigher limit value PBAH (e.g. 610 mmHg). If the answer to this questionis affirmative (YES), it is determined at a step S5 whether or not thethrottle valve opening θTH detected by the θTH sensor 4 falls between apredetermined lower limit value θTHL (e.g. 1°) and a higher limit valueθTHH (e.g. 5°). If the answer to this question is affirmative (YES), itis determined at a step S6 whether or not the vehicle speed VSP detectedby the VSP sensor 21 falls between a predetermined lower limit value(eg. 53 Km/h) and a predetermined higher limit value (e.g. 61 Km/h). Ifthe answer to this question is affirmative (YES), it is judged that theengine 1 has been warmed up and at the same time is in a stableoperating condition, so that the program proceeds to a step S7.

At the step S7, it is determined whether or not the vehicle on which theengine 1 is installed is cruising. This determination of cruising of thevehicle is carried out by determining whether or not the vehicle hascontinued to travel with a change in the vehicle speed being equal to orsmaller than a value of ±0.8 Km/sec. over two seconds. If the answer tothis question is affirmative (YES), it is determined at a step S8whether or not the PT sensor 29, and the first to third control valves28, 36, 40 are normally operating. If the answer to this question isaffirmative (YES), it is determined at a step S9, from the output fromthe hot-wire type flowmeter 37, whether or not the purging flow rate ofa mixture of evaporative fuel and air flowing through the purgingpassage 10 shows a sufficient value. If the answer to this question isaffirmative (YES), it is judged that the monitoring conditions aresatisfied, so that a flag FMON is set to “1” at a step S10, followed byterminating the program. On the other hand, if at least one of theanswers to the questions of the steps S1 to S9 is negative (NO), it isjudged that the monitoring conditions are not satisfied, so that theflag FMON is set to “0” at a step S11, followed by terminating theprogram.

FIG. 6 shows a program for carrying out the abnormality diagnosis of theevaporative emission control system 11, which is executed by the ECU 5of the evaporative fuel-processing system according to a firstembodiment of the invention. This program is executed as backgroundprocessing.

First, at a step S21, it is determined whether or not the flag FMON hasbeen set to “1” in the monitoring condition-determining routinedescribed above with reference to FIG. 5. Immediately after the engine 1has been started, the monitoring conditions are not satisfied, and hencethe answer to the question of the step S21 is negative (NO), so that theprogram proceeds to a step S22, where a first timer tmPTO, formed of adown-counter, is set to a predetermined time period T1, and started. Thefirst timer tmPTO is provided to secure a sufficient time period forstabilizing the tank internal pressure PT after the tank internalpressure PT is relieved to the atmosphere, and accordingly thepredetermined time period T1 assumes a value of 30 sec., for example.After the first timer tmPTO is started, the program proceeds to a stepS23, where the evaporative emission control system 11 is set to thenormal purging mode, i.e. the first and second electromagnetic valves35, 39 are turned off and at the same time the second control valve 36is turned on as shown at (i) in FIG. 4, followed by terminating theprogram.

If the monitoring conditions are satisfied in a subsequent loop, theflag FMON is set to “1”, and hence the answer to the question of thestep S21 becomes affirmative, so that the program proceeds to a stepS24, where it is determined whether or not the count value of the firsttimer tmPTO has become equal to “0” to determined whether thepredetermined time period T1 has elapsed. In the first execution of thestep S24, the answer to this question is negative (NO), so that theprogram proceeds to a step S25, where the system 11 is set to theopen-to-atmosphere mode. That is, as described hereinbefore (at the timeperiod indicated by (ii) in FIG. 4), the first electromagnetic valve 35and the second control valve 36 are held energized, and at the same timethe second electromagnetic valve 39 is held deenergized. Then, a secondtimer tmPTD, formed of an up counter, is set to “0” at a step S26. Thesecond timer tmPTD is provided to measure a time period elapsed beforethe negatively-pressurized condition of the evaporative emission controlsystem 11 is established, as described hereinafter. The timer tmPTD isinitially set to “0”. Then, the tank internal pressure PTO assumed whenthe system 11 is in the open-to-atmosphere condition is set to a presentvalue of the tank internal pressure PT detected by the PT sensor 29 at astep S27, and a flag FRDC, which is set to “1” when thenegatively-pressurizing process is completed, is set to “0” at a stepS28, followed by terminating the program. That is, the tank internalpressure PTO in the open-to-atmosphere condition is renewed to a presentvalue of the PT, and the flag FRDC is reset, followed by terminating theprogram.

When the predetermined time period T1 has elapsed to make the countvalue of the first timer tmPTO equal to “0”, in a subsequent loop, theanswer to the question of the step S24 becomes affirmative (YES), sothat the program proceeds to a step S29, where it is determined whetheror not the flag FRDC is equal to “1”. In the first execution of the stepS29, the answer to this question is negative (NO), so that the programproceeds to a step S30, where it is determined whether or not the tankinternal pressure PT is equal to or lower than a predetermined referencevalue PTLVL (e.g. −20 mmHg). In the first execution of the step S30, theevaporative emission control system 11 is in the open-to-atmospherecondition, and hence the inside-tank pressure PT is substantially equalto the atmospheric pressure, so that the answer to the question of thestep S30 is negative (NO), and accordingly the program proceeds to astep S31 where the evaporative emission control system 11 is negativelypressurized. More specifically, as described hereinbefore with referenceto FIG. 4 (see the time period (iii) in FIG. 4), the first and secondelectromagnetic valves 35, 39 and the second control valve 36 are allturned on or energized to create negative pressure within theevaporative emission control system 11. Then, at a step S32, the secondtimer tmPTD is set to a time period T2 required to create negativepressure within the system 11, i.e. a time period T2 elapsed after itwas set to “0” at the step S26. The program then proceeds to a step S33,where a third timer tmPTDC, formed of a down counter, for leak downcheck is set to a predetermined time period T3, followed by terminatingthe program. The predetermined time period T3 assumes a value of e.g. 30sec. which will be required for completing the leak down check.

When the negatively-pressurized condition of the evaporative emissioncontrol system 11 necessary for the leak down check is established, andhence the answer to the question of the step S30 becomes affirmative(YES), the flag FRDC is set to “1” at a step S34, and then the programproceeds therefrom to a step S35, where it is determined whether or notthe count value of the third timer tmPTDC is equal to “0” to judgewhether the time period required for completing the leak down check haselapsed.

In the first execution of the step S35, the answer to the question ofthe step S35 is negative (NO), so that the program proceeds to a stepS36, where a fourth timer tmPDTDCS for correcting the leak down check isset to a predetermined time period T4. The correcting time period T4 iscalculated based on conditions of the fuel tank 23 (fuel amount, fueltemperature, tank internal pressure, negatively-pressurizing timeperiod), and provided to retard abnormality diagnosis to be performed ata step S39, described hereinafter. The reason for retarding the timingfor execution of abnormality diagnosis depending on the conditions ofthe fuel tank 23 is as follows:

When the fuel tank 23 is substantially fully filled with fuel, thevolume of space above fuel of the fuel tank 23 is small, so that thetank internal pressure PT increases at a higher speed as is obvious fromFIG. 3, whereas when the amount of fuel contained in the fuel tank 23 issmall, the tank internal pressure PT increases at a lower speed, afterestablishment of the negatively-pressurized condition of the evaporativeemission control system 11. Therefore, depending on the amount of fuelcontained in the fuel tank 23, there can be made an erroneousdetermination as to abnormality of the system 11. Further, if a longertime period is required in establishing the negatively-pressurizedcondition of the system 11, it takes a longer time period to completethe leak down check, and therefore it may be required to modify themanner of determining abnormality depending on the time period requiredin establishing the negatively-pressurized condition of the system 11.Further, when the fuel temperature is high, the amount of evaporativefuel generated within the fuel tank 23 is large, so that the tankinternal pressure PT increases at a higher speed, which can lead to anerroneous detection of abnormality of the system 11. Further, when thetank internal pressure in the open-to-atmosphere condition is high,which means the atmospheric pressure outside the system is high it takesa short time period for the tank internal pressure PT, after the systemhas been negatively pressurized, to rise to a predetermined referencevalue, mentioned hereinafter, which can result in an erroneous detectionof abnormality of the system 11. Therefore, in order to prevent sucherroneous determinations of abnormality, the timing for starting theexecution of abnormality determination is corrected depending on theconditions of the fuel tank 23.

More specifically, the correcting time period T4 is calculated by theuse of the following equation (1):

T4=ΔTTF+ΔTVF+ΔTPTO+ΔTtmPTD . . .   (1)

where ΔTTF represents a fuel temperature-dependent correcting timeperiod, which is calculated by retrieving a ΔTTF map stored in thememory means of the ECU 5. The ΔTTF map can be set, e.g. as shown inFIG. 7, such that predetermined values ΔTTF0 to ΔTTF3 are providedcorresponding, respectively, to predetermined fuel temperature valuesTF0 to TF3. A value of the correcting time period ΔTTF is read from theΔTTF map or calculated by interpolation.

ΔTVF represents a fuel amount-dependent correcting time period, which iscalculated by retrieving a ΔTVF map stored in the memory means of theECU 5. The ΔTVF map can be set, e.g. as shown in FIG. 8, such thatpredetermined values ΔTVF0 to ΔTVF3 are provided corresponding,respectively, to predetermined fuel amount values VF0 to VF3. A value ofthe correcting time period ΔTVF is read from the ΔTVF map or calculatedby interpolation.

ΔTPTO represents a tank internal pressure-dependent correcting timeperiod, which is calculated by retrieving a ΔTPTO map stored in thememory means of the ECU 5. The ΔTPTO map can be set, e.g. as shown inFIG. 9, such that predetermined values ΔTPTO0 to ΔTPTO3 are providedcorresponding, respectively, to predetermined tank internal pressurevalues in the open-to-atmosphere condition PTO0 to PTO3. A value of thecorrecting time period ΔTPTO is read from the ΔTPTO map or calculated byinterpolation.

ΔTmPTD represents a negatively-pressurizing time period-dependentcorrecting time period, which is calculated by retrieving a ΔTtmPTD mapstored in the memory means of the ECU 5. The ΔTtmPTD map can be set,e.g. as shown in FIG. 10, such that predetermined values ΔTtmPTD0 toΔTtmPTD3 are provided corresponding, respectively, to predeterminednegatively-pressurizing time periods tmPTD0 to tmPTD3. A value of thecorrecting time period ΔTtmPTD is read from the ΔTtmPTD map orcalculated by interpolation

As is clear from FIGS. 7 to 10, the correcting time periods ΔTTF, ΔTVFand ΔTPTO are set to smaller values as the fuel temperature TF, the fuelamount FV, and the tank internal pressure PTO assume higher, larger andhigher values, respectively, while ΔTtmPTD is set to a larger value asnegatively-pressurizing time period tmPTD assumes a larger value.

Thus, the fourth timer tmPTDCS is set to the correcting time period T4calculated by the use of the equation (1), and then the evaporativeemission control system 11 is set to the leak down check mode at a stepS37, followed by terminating the program. More specifically, asdescribed hereinbefore with reference to FIG. 4 (see the time period 4in FIG. 4), the first and second electromagnetic valves 35, 39 are heldON or energized, respectively, and at the same time the second controlvalve 36 is turned off or deenergized, followed by terminating theprogram. In this connection, when the negatively-pressurizing process iscompleted, the flag FRDC is set to “1”, and hence the answer to thequestion of the step S29 becomes affirmative (YES), so that the step S35is immediately carried out.

When the answer to the question of the step S35 is affirmative (YES),the program proceeds to a step S38, where it is determined whether ornot the correcting time period T4 has elapsed and hence the count valueof the fourth timer tmPTDCS is equal to “0”. If the answer to thisquestion is negative (NO), the program proceeds to the step S37, wherethe leak down check is continued, followed by terminating the program.On the other hand, if the answer to the question of the step S38 isaffirmative (YES), the program proceeds to a step S39, where anabnormality-determining routine is executed, and then the evaporativeemission control system 11 is restored to the normal purging mode at thestep S23, followed by terminating the program.

FIG. 11 shows an example (Abnormal Determination A) of theabnormality-determining routine executed at the step S39 (in FIG. 6).

At a step S41, it is determined whether or not the internal tankpressure PT is higher than a reference value PTJDG (e.g. −10 mmHg). Ifthe answer to this question is affirmative (YES), it is judged that theevaporative emission control system 11 suffers from a significantleakage and hence it is determined that the system is in an abnormalcondition, at a step S42, followed by returning to the main routine ofFIG. 6. On the other hand, if the answer to the question of the step S41is negative (NO), it is judged that no leakage occurs in the system 11,and hence it is determined that the system is in a normal condition, ata step S43, followed by returning to the main routine of FIG. 6.

FIG. 12 shows another example (Abnormal Determination B) of theabnormality-determining routine.

First, at a step S51, a calculation is made of a rate of change ΔPTD inthe internal tank pressure PT (hereinafter referred to as “the pressurereduction rate”) occurring when the evaporative emission control system11 is negatively-pressurized to a predetermined value PTLVL, i.e. thenegatively-pressurized condition thereof is established, by the use ofthe following equation (2). More specifically, an amount of change inthe internal tank pressure PT in establishing the negatively-pressurizedcondition of the evaporative emission control system 11 is divided bythe time period T2 required for the tank internal pressure to be reducedto the predetermined value from the tank internal pressure PTO in theopen-to-atmosphere condition, to calculate the pressure reduction rateΔPTD.

ΔPTD=(PTO−PTLVL)/T2 . . .   (2)

Further, a calculation is made of a rate of change ΔPTL in theinside-tank pressure PT (hereinafter referred to as “leakage rate”)occurring after the negatively-pressurized condition of the system hasbeen established by the use of the following equation (3). Morespecifically, an amount of change in the inside-tank pressure PToccurring after the aforementioned condition of the system 11 has beenestablished is divided by a time period required for the leak down check(i.e. the sum of the time period T3 and the correcting time period T4)to obtain the leakage rate ΔPTL.

ΔPTL=(PT−PTLVL)/(T3+T4). . .   (3)

Then at a step S52, the ratio of the leakage rate ΔPTL to the pressurereduction rate ΔPTD is calculated, and it is determined the ratiocalculated is larger than a predetermined reference value PTRJDG. If theanswer to this question is affirmative (YES), it is judged that theleakage is significant, and hence is determined that the system 11 is inan abnormal condition, at a step S53, followed by returning to the mainroutine of FIG. 6. On the other hand, if the answer to the question ofthe step S52 is negative (NO), it is judged that the leakage isinsignificant, and hence it is determined that the system 11 is in anormal condition, at a step S54, followed by returning to the mainroutine of FIG. 6.

As described above, according to the present embodiment, the evaporativeemission control system 11 is negatively pressurized, and then in thisstate, it is determined based the behavior of on the tank internalpressure PT whether or not the evaporative emission control system 11 isin a normal condition. Therefore, it is possible to detect deteriorationin the seals provided at the piping connections, the fuel tank 23, etc.,which enables to prevent evaporative fuel from being emitted into theair.

Further, since the timing for determining abnormality of the system 11is corrected based on conditions of the fuel tank (fuel amount, fueltemperature, etc.), it is possible to achieve even more accurateabnormality determination.

FIG. 13 shows changeovers of operative states of the first and secondelectromagnetic valves 35, 39, the drain shut valve 38, and the secondcontrol valve 36 of the system, and changes in the inside-tank pressurePT resulting therefrom, according to a second embodiment of theinvention. The operative states of the valves are changed over byrespective corresponding signals supplied from the ECU 5 (CPU).

Under the normal operating condition (in the normal purging mode),during a time period indicated by (i) in FIG. 13, the firstelectromagnetic valve 35 is energized, while the second electromagneticvalve 39 is deenergized. When the ignition switch IGSW is closed and theIGSW sensor detects that the engine 1 is in operation, the purge controlvalve 36 is turned on or opened. Evaporative fuel generated in the fueltank 23 then flows via the evaporative fuel-guiding passage 27 into thecanister 26, where it is temporarily adsorbed by the adsorbent 24.Further, since the second electromagnetic valve 39 is in the deenergizedstate under the normal operating condition as mentioned above, the drainshut valve 38 is open to allow the outside air to be supplied to thecanister 26 via the air-introducting port 45 a. Accordingly, theevaporative fuel flowing into the canister 26 is purged together withthe air thus introduced, via the second control valve 36 through thepurging passage 10 into the intake pipe 2. In this connection, ifnegative pressure within the fuel tank 23 increases due to coolingthereof caused by the outside air, etc., the negative pressure valve 33of the two-way valve 34 is opened to return evaporative fuel stored inthe canister 26 to the fuel tank 3.

When predetermined monitoring conditions, described in detailhereinafter, are satisfied, the first and second electromagnetic valves35, 39, and the purge control valve 36 are operated in the followingmanner to carry out an abnormality diagnosis of the evaporative emissioncontrol system 11.

First, the tank internal pressure PT is relieved to the atmosphere, overa time period indicated by (ii) in FIG. 13. More specifically, the firstelectromagnetic valve 35 is held in the energized state to maintaincommunication between the fuel tank 23 and the canister 26, and at thesame time the second electromagnetic valve 39 is held in the deenergizedstate to keep the drain shut valve 38 open. Further, the purge controlvalve 36 is held in the energized state or opened, to relieve the tankinternal pressure PT to the atmosphere.

Then, an amount of change in the tank internal pressure PT is measuredover a time period indicated by (iii) in FIG. 13.

More specifically, the second electromagnetic valve 39 is held in thedeenergized state to keep the drain shut valve 38 open, and at the sametime the purge control valve 36 is kept open. However, the firstelectromagnetic valve 35 is turned off into the deenergized state, tothereby measure an amount of change in the tank internal pressure PToccurring after the fuel tank 23 has ceased to be open to the atmospherefor the purpose of checking an amount of evaporative fuel generated inthe fuel tank 23.

Then, the evaporative emission control system 11 is negativelypressurized over a time period indicated by (iv) in FIG. 13. Morespecifically, the first electromagnetic valve 35 and the purge controlvalve 36 are held in the energized state, while the secondelectromagnetic valve 39 is turned on to close the drain shut valve 38,whereby the evaporative emission control system 11 is negativelypressurized by a gas-drawing force developed by negative pressure in thepurging passage 10 held in communication with the intake pipe 2. In thefigure, TR represents a time period required for establishing thenegatively-pressurized condition of the system.

Then, a leak down check is carried out over a time period indicated by(v) in FIG. 13.

More specifically, after the evaporative emission control system 11 isnegatively pressurized to a predetermined degree, i.e. after thenegatively-pressurized condition of the system is established, the purgecontrol valve 36 is closed, and then a change in the tank internalpressure PT occurring thereafter is checked by the PT sensor 29. If thesystem 11 suffers from no significant leak of evaporative fueltherefrom, and hence the result of the leak down check shows that thereis substantially no change in the tank internal pressure PT as indicatedby the two-dot-chain line in the figure, it is judged that theevaporative emission control system 11 is normal, whereas if the system11 suffers from a significant leak of evaporative fuel therefrom, andhence the result of the leak down check shows that there is asignificant change in the tank internal pressure PT toward theatmospheric pressure it is judged that the system 11 is abnormal.Further, if the evaporative emission control system 11 cannot attain thenegatively-pressurized condition within a predetermined time period, theleak down check is not carried out, as described hereinafter.

After determining whether or not the system 11 is normal, the system 11returns to the normal purging mode, as indicated by (vi) in FIG. 13.

More specifically, while the first electromagnetic valve 35 is held inthe energized state, the second electromagnetic valve 39 is deenergizedand the purge control valve 36 is opened, to thereby perform normalpurging of evaporative fuel. In this state, the tank internal pressurePT is relieved to the atmosphere, and hence is substantially equal tothe atmospheric pressure.

Next, there will be described, with reference to related figures, themanner of abnormality diagnosis of the evaporative fuel-processingsystem according to the second embodiment of the invention.

FIG. 14 shows a program for carrying out the abnormality diagnosis ofthe evaporative emission control system 11, which is executed by the ECU5 (CPU).

First at a step S101, a routine of determining permission for monitoringis carried out, as described hereinafter. Then, at a step S102, it isdetermined whether or not the monitoring of the system 11 forabnormality diagnosis is permitted, i.e. a flag FMON is set to “1”, atthe step S101. If the answer to this question is negative (NO), thefirst to third control valves 28, 36, 40 are set to respective operativestates for the normal purging mode of the system, followed byterminating the program, whereas if the answer to this question isaffirmative (YES), the tank internal pressure PT in theopen-to-atmosphere condition of the system is checked at a step S103,and it is determined at a step S104 whether or not this check has beencompleted. If the answer to this question is negative (NO), the programis immediately terminated, whereas if it is affirmative (YES), i.e. ifit is judged that the above check has been completed, the firstelectromagnetic valve 35 is turned off to check a change in the tankinternal pressure PT at a step S105, followed by determining at a stepS106 whether or not this check has been completed. If the answer to thisquestion is negative (NO), the program is immediately terminated,whereas if it is affirmative (YES), the first to third control valves28, 36, 40 are operated at a step S107 to establish thenegatively-pressurized condition of the evaporative emission controlsystem 11 including the fuel tank 23.

Simultaneously with the start of the negatively pressurizing process atthe step S107, a first timer tmPRG incorporated in the ECU5 is started,and it is determined at a step 108 whether or not the count valuethereof is larger than a value corresponding to a predetermined timeperiod T5. The predetermined time period T5 is set to such a value aswill ensure that the system 11 is negatively pressurized to apredetermined pressure value, i.e. the negatively-pressurized conditionof the system 11 is established, if the system is normal. If the answerto the question of the step S108 is affirmative (YES), it is judged thatthe system 11 cannot be negatively pressurized to the predeterminedpressure value due to a hole formed in the fuel tank 23, etc., theprogram proceeds to a step S112. On the other hand, if the answer to thequestion of the step S109 is negative (NO), it is determined at a stepS109 whether or not the negatively-pressurizing process has beencompleted, i.e. the negatively-pressurized condition of the system 11 isestablished. If the answer to this question is negative (NO), theprogram is immediately terminated, whereas if it is affirmative (YES), aleak down check routine, described in detail hereinafter, is carried outat a step S110 to check whether or not the system 11 is properly sealed,i.e. it is free from a leak of evaporative fuel therefrom in the normaloperating mode thereof. Then, at a step S111, it is determined whetheror not this check has been completed.

If the answer to this question is negative (NO), the program isimmediately terminated, whereas if the answer is affirmative (YES), theprogram proceeds to a step S112.

At the step S112, a process is carried out for determining whether ornot the system 11 is in a normal condition, followed by determining at astep S113 whether this process has been completed. If the answer to thisquestion is negative (NO), the program is immediately terminated,whereas if it is affirmative (YES), the system 11 is set to the normalpurging mode at a step S114, followed by terminating the program.

Next, the above steps will be described in detail.

(1) Determination of permission for monitoring (at the step S101 of FIG.14)

FIG. 15 shows a routine for determining whether or not monitoring of thesystem 11 for abnormality diagnosis thereof is permitted. This routineis executed as background processing Steps S122 to S123 of this programare identical to the steps S1 to S7 of the program of FIG. 6.

At a step S121, it is determined whether or not the engine coolanttemperature TWI is lower than a predetermined value TWX. The abnormalitydiagnosis of the present embodiment has only to be carried out onlyafter the engine has been out of operation for a long time period (e.g.once per day). First, when the ignition switch IGSW is closed, theengine coolant temperature TWI at the start of the engine is detectedand read in, and it is determined at the step S121 in the presentroutine whether or not the engine coolant temperature TWI is lower thanthe predetermined value, e.g. 20° C. If the answer to this question isaffirmative (YES), i.e. if the engine coolant temperature TWI at thestart of the engine is lower than the predetermined value TWX, theprogram proceeds to a step S122.

At the steps S122 to S128, determinations identical to those of thesteps S1 to S7 are carried out. If the answer to the question of thestep S128 is affirmative (YES), it is determined at a step S129 whetheror not purging of evaporative fuel has been carried out over apredetermined time period. More specifically, in the case where a largeamount of evaporative fuel is stored in the canister 26, it takes alonger time period to establish the negatively-pressurized condition ofthe system 11 due to the resulting large resistance of the canister 26to permeation of gases or there is a fear that unpreferably richevaporative fuel be purged into the intake system during thenegatively-pressurizing process. Therefore, in the present embodiment,monitoring of the evaporative emission control system 11 is carried outonly after the purging of evaporative fuel has been carried over thepredetermined time period, to reduce the amount of evaporative fueladsorbed and stored in the canister 26.

If the answer to the question of the step S129 is affirmative (YES), theprogram proceeds to a step S130, where it is determined whether or notthe fuel temperature TF of fuel contained in the tank 23 detected by theTF sensor 31 is lower than a predetermined value TFH (e.g. 35° C.).

If the answer to this question is affirmative (YES), the flag FMON isset to “1” at a step S131 for permitting monitoring of the system 12 forabnormality diagnosis, followed by terminating the program. On the otherhand, if at least one of the answers to the questions of the steps S121to S130 is negative (NO), the conditions for permitting monitoring arenot satisfied, so that the flag FMON is set to “0” at a step S132,followed by terminating the program.

The step S129 is provided in consideration of the fact that theabnormality determination, described hereinafter, cannot be accuratelycarried out in the case where the fuel temperature TF is higher than thepredetermined value (i.e. 35° C.). By inhibiting the monitoring when thefuel temperature TF is high, it is possible to avoid an erroneousdetermination of abnormality of the system 11. This will be furtherexplained in detail hereinafter.

(2) Check of the tank internal pressure in the open-to atmospherecondition (at the step S103 in FIG. 14)

FIG. 16 shows a routine for carrying out the tank internal pressurecheck in the open-to-atmosphere condition, which is also executed asbackground processing.

First, at a step S141, the system 11 is set to the open-to-atmospheremode, and at the same time, a second timer tmATMP is started. Morespecifically, the first electromagnetic valve 35 is held in theenergized state, and at the same time the second electromagnetic valve39 is held in the deenergized state to keep the drain shut valve 38open. Further, the purge control valve 36 is kept open. Thus, the tankinternal pressure PT is relieved to the atmosphere (See the time periodindicated by (ii) in FIG. 13).

Then, at a step S142, it is determined whether or not the count value ofthe second timer tmATMP is larger than a value corresponding to apredetermined time period T6. The predetermined time period T6 is set toa value, e.g. 4 sec., which ensures that the pressure within the system11 has been stabilized upon lapse thereof. If the answer to thisquestion is negative (NO), the program is immediately terminated, whileif it is affirmative (YES), the program proceeds to a step S143, wherethe tank internal pressure PATM in the open-to-atmosphere condition isdetected by the PT sensor 29 and stored in the ECU 5, and then acheckover flag is set at a step S144, followed by terminating theprogram.

(3) Check of a change in the tank internal pressure (at the step S105 inFIG. 14)

FIG. 17 shows a routine for checking a change in the tank internalpressure, which is executed as background processing.

First, at a step S151, the system 11 is set to a PT change-checkingmode, and at the same time a third timer tmTP is started. Morespecifically, while the purge control valve 36 and the drain shut valve38 are held open, the first electromagnetic valve 35 is turned off tothereby set the system to the PT change-checking mode (See the timeperiod indicated by (iii) in FIG. 13).

Then, at a step S152, it is determined whether or not the count value ofthe third timer tmTP is larger than a value corresponding to apredetermined time period T7, e.g. 10 sec. If the answer to thisquestion is negative (NO), the program is immediately terminated,whereas if it is affirmative (YES), the tank internal pressure PCLSafter the lapse of the predetermined time period T7 is detected andstored in the ECU 5 at a step S153, followed by calculation of a firstrate of change PVARIA in the tank internal pressure by the use of thefollowing equation (4):

PVARIA=(PCLS−PATM)/T3 . . .   (4)

Then, the first rate of change PVARIA thus calculated is stored in theECU 5 and a check-over flag is set at a step S155, followed byterminating the program.

(4) Negatively pressurizing process (at the step S107 in FIG. 14)

FIG. 18 shows a routine for carrying out a process of negativelypressurizing the system 11 to establish the negatively-pressurizedcondition of the system, which is executed as by background processing.

First, at a step S161, the system 11 is set to a negatively-pressurizingmode. More specifically, the purge control valve 36 is kept open, and atthe same time the first electromagnetic valve 35 is held in theenergized state, and the second electromagnetic valve is turned on toclose the drain shut valve 38 (see the time period indicated by (iv) inFIG. 13). In this state, the system 11 is negatively pressurized to apredetermined value by a gas-drawing force created by operation of theengine 1. Then, it is determined at a step S162 whether or not the tankinternal pressure PCHK in this mode of the system 11 is lower than apredetermined value PI (e.g. −20 mmHg). If the answer to this questionis negative (NO), the program is immediately terminated, whereas if itbecomes affirmative (YES), a processor flag is set at a step S63,followed by terminating the program.

(5) Leak down check (at the step S110 in FIG. 14)

FIG. 19 shows a routine for performing a leak down check of the system11, which is executed as background processing.

First, at a step S171, the system 11 is set to a leak down check mode.More specifically, while the first electromagnetic valve 35 is held inthe energized state, and at the same time the drain shut valve is keptclosed, the purge control valve 36 is closed to cut off thecommunication between the system 11 and the intake pipe 2 of the engine1 (see the time period (v) in FIG. 13).

Then, the program proceeds to a step S172, where it is determinedwhether or not the tank internal pressure PST at the start of the leakdown check has been detected. In the first execution of this step S172,the answer to this question is negative (NO), so that the programproceeds to a step S173, where the tank internal pressure PST isdetected and a fourth timer tmLEAK is started.

Then, it is determined at a step S174 whether or not the count value ofthe fourth timer tmLEAK is larger than a value corresponding to apredetermined time period T8 (e.g. 10 sec.). In the first execution ofthis step S172, the answer to this question is negative (NO), so thatthe program is immediately terminated.

In the following loop, the answer to the question of the step S172becomes affirmative (YES), so that the program jumps over to the stepS174, where it is determined whether or not the count value of thefourth timer tmLEAK is larger than the value corresponding to thepredetermined time period T8. If the answer to this question is negative(NO), the program is immediately terminated, whereas if it becomesaffirmative (YES), the present tank internal pressure i.e. the tankinternal pressure PEND at the end of the leak down check is detected andstored into the ECU 5 at a step S175, followed by calculation of asecond rate of change PVARIB in the tank internal pressure PT at a stepS176 by the use of the following equation (5):

PVARIB=(PEND−PST)/T4 . . .   (5)

The second rate of change PVARIB in the tank internal pressure PT thuscalculated is stored into the ECU 5, and a check-over flag is set at astep S177, followed by terminating the program.

(6) System condition-determining process (at the step S112 in FIG. 14)

FIG. 20 shows a routine for carrying out a process of determining acondition of the system 11, which is executed as by backgroundprocessing.

First, at a step S181, it is determined whether or not the count valueof the first timer tmPRG exceeded the predetermined value T5 during thenegatively-pressurizing process. If the answer to this question isaffirmative (YES), it is judged that the system 11 may suffer from asignificant leak of evaporative fuel due to a hole formed in the fueltank 23, etc., so that the program proceeds to a step S182, where it isdetermined whether or not the first rate of change PVARIA in the tankinternal pressure PT is larger than a predetermined value P2. If theanswer to this question is negative (NO), which means that evaporativefuel was not generated at a large rate in the fuel tank 23, and hencethe negatively-pressurized condition of the system 11 could have beenproperly established in the negatively-pressurizing process if thesystem 11 had been in a normal condition, it is judged that the system11 suffers from a significant leak of evaporative fuel from the fueltank 23, piping connections, etc., determining that the evaporativeemission control system 11 is abnormal, and then a process-over flag isset at a step S136, followed by terminating the program. On the otherhand, if the answer to the question of the step S182 is affirmative(YES), which means that evaporative fuel was generated at a large ratein the fuel tank 23 to increase the tank internal pressure PT, whichprevented the system 11 from being negatively pressurized in a propermanner in the negatively-pressurizing process, the determination of thesystem condition is suspended at a step S184, and then the process overflag is set at the step S186, followed by terminating the program.

On the other hand, if the answer to the question of the step S181 isnegative (NO), i.e. if the system 11 was negatively pressurized to thepredetermined value, an abnormality determining routine is carried outat a step 185, and then the process-over flag is set at the step S186,followed by terminating the program.

The abnormality-determining routine carried out at the step S185 isshown by way of example in FIG. 21.

First, it is determined at a step S191 whether or not the differencebetween the second change of rate PVARIB in the tank internal pressurePT and the first rate of change PVARIA in same is larger than apredetermined value P3.

More specifically, in order to determine whether a main factor which hasdetermined the rate of change PVARIB in the tank internal pressure PT isthe faulty scaling of the system 11, which means that there occurs asignificant leak of evaporative fuel from the system 11 in the normaloperating mode thereof, or generation of evaporative fuel from the fueltank 23, it is determined whether or not the difference between thesecond rate of change PVARIB and the first rate of change PVARIA islarger than the predetermined value P3. If the second rate of changePVARIB assumes a large value due to generation of a large amount ofevaporative fuel from the fuel tank 23, the answer to the question ofthe step S191 is negative (NO), whereas if the second rate of changePVARIB assumes a large value due to the faulty sealing of the system 11,the answer is affirmative (YES). The predetermined value P3 is setaccording to the time period TR required for establishing thenegatively-pressurized condition of the system 11 in a manner as shownin FIG. 22. More specifically, the predetermined value P3 is set to avalue P31 when the time period TR is longer than a predetermined valueTR1, whereas it is set to a value P32 (>P31) when the time period TR isshorter than the predetermined value TR1. If the answer to the questionof the step S191 is affirmative (YES), it is determined at a step S192that the evaporative emission control system 11 is abnormal, whereas ifthe answer is negative (NO), it is determined at a step S193 that thesystem 11 is normal, followed by terminating the program.

FIG. 23 shows another example of the abnormality-determining routine.

First, at a step S201, it is determined whether or not the fuel amountFV in the fuel tank 23 detected by the FV senor 30 is larger than afirst predetermined value FV1, to determine whether or not the fuel tank23 is Substantially fully filled with fuel. If the answer to thisquestion is affirmative (YES), a map [I] is selected, whereas if theanswer is negative (NO), it is determined at a step S203 whether or notthe fuel amount FV is larger than a second predetermined value FV2, todetermine whether or not the fuel tank 23 is filled half or more withfuel. If the answer to this question is affirmative (YES), a map [II] isselected at a step S204, whereas if the answer is negative (NO), a map[III] is selected at a step S205.

Then, the abnormality-determination is carried out by the use of aselected one of the maps [I] to [III], followed by terminating theprogram.

More specifically, as shown in FIGS. 24 [I]-[III], the maps [I] to [III]are each formed such that a normal region and an abnormal region aredefined in a manner depending on the relationship between the first rateof change PVARIA in the tank internal pressure PT and the second rate ofchange PVARIB in the tank internal pressure PT. By retrieving theselected one of the maps, it is determined whether or not the system 11is normal. In the figures, the hatched sections indicate the abnormalregions.

(7) Normal purging (at the step S114 in FIG. 14)

FIG. 25 shows a routine for restoring the normal purging mode of thesystem 11, in which the operative states of the valves are specified.

More specifically, the first electromagnetic valve 35 is held in theenergized state and the drain shut valve 39 and the purge control valve36 are opened to thereby set the system to the normal purging mode, at astep S211, followed by terminating the program.

As described heretofore, according to the present embodiment, if thepredetermined time period T5 has elapsed during the process ofnegatively-pressurizing the system 11, it is immediately determined (byjumping-over of the step S108 to S112 in FIG. 14) whether or not thesystem 11 is abnormal. Therefore, even if the system 11 cannot benegatively pressurized to the predetermined value, it is possible todetermine whether or not the system 11 is abnormal.

Further, according to the present embodiment, as shown in FIG. 21 orFIG. 23, the abnormality determination of the system is carried out withreference to the relationship between the first rate of change PVARIA inPT calculated during the PT change check (at the step S105 in FIG. 14;and FIG. 17) and the second rate of change PVARIB in PT calculatedduring the leak down check (at the step S110 in FIG. 14; and FIG. 19),it is possible to perform an accurate abnormality determination even ifevaporative fuel is being generated at a large rate. That is, it can beavoided to erroneously determine that the system is abnormal whenevaporative fuel is generated at a large rate.

Further, when the fuel temperature TF is at a normal value (20° C.), therelationship between the first rate of change PVARIA and the second rateof change PVARIB has a marked border line between the normal region andthe abnormal region as shown in FIG. 26a depending on whether the systemsuffers from a leak or not, and hence, it is possible to effect accuratedetermination of abnormality of the system by the use of a referencelevel indicated in the figure. However, when the fuel temperature TF ishigh, e.g. 40° C., the marked border line cannot be discriminated fromthe relationship between the first and second rates of changes resultingfrom whether the system suffers from a leak of evaporative fuel or not,making it impossible to effect accurate abnormality determination.Therefore, by the step S130 in FIG. 15, the abnormality determination isinhibited when the fuel temperature TF is high (>TFH), to therebyprevent an erroneous determination of abnormality, which enhances theaccuracy of the abnormality determination.

Although, in the above embodiments of the invention, the third controlvalve 40 is comprised of the drain shut valve 38, the secondelectromagnetic valve 39, and the negative pressure communicationpassage 9, this is not limitatine, but the third control valve 40 may beconstituted by a single electromagnetic valve 60 for opening and closingthe air inlet port 25 to control introduction of air into the consister26. This contributes to simplification of the construction of theevaporative fuel-processing system of the invention.

What is claimed is:
 1. An evaporative fuel-processing system for aninternal combustion engine having an intake system, including anevaporative emission control system having a fuel tank, a canistercontaining an adsorbent, said canister having an air inlet portcommunicatable with the atmosphere, an evaporative fuel-guiding passageextending between said canister and said fuel tank, a first controlvalve arranged across said evaporative fuel guiding passage, anevaporative fuel-purging passage extending between said canister andsaid intake system, and a second control valve arranged across saidevaporative fuel-purging passage, said evaporative fuel-processingsystem having an abnormality-determining system in which comprises:pressure-detecting means for detecting pressure within said evaporativeemission control system; negatively-pressurizing means for negativelypressurizing said evaporative emission control system; andabnormality-determining means for determining abnormality of saidevaporative emission control system based on the pressure within saidfuel tank detected after said evaporative emission control system hasbeen negatively pressurized by said negatively-pressurizing means.
 2. Anevaporative fuel-processing system according to claim 1, wherein saidabnormality-determining means determines the abnormality of saidevaporative emission control system based on a rate of change in thepressure within said fuel tank occurring before said evaporativeemission control system is set to a predetermined negatively-pressurizedcondition by said negatively-pressurizing means and a rate of change inthe pressure within said fuel tank occurring after said predeterminednegatively-pressurized condition of said evaporative emission controlsystem has been established.
 3. An evaporative fuel-processing systemaccording to claim 1, including tank condition-detecting means fordetecting conditions of said fuel tank, wherein saidabnormality-determining means carries out abnormality determination whena predetermined time period has elapsed after said evaporative emissioncontrol system was negatively pressed said predetermined time periodbeing corrected by a correcting time period set in response to saidconditions of said fuel tank detected by said tank condition-detectingmeans.
 4. An evaporative fuel-processing system according to claim 2,including tank condition-detecting means for detecting conditions ofsaid fuel tank, wherein said abnormality-determining means carries outabnormality determination when a time period has elapsed after saidevaporative emission control system was negatively pressurized, saidpredetermined time period being corrected by a correcting time periodset in response to said conditions of said fuel tank detected by saidtank conditioned-detecting means.
 5. An evaporative fuel-processingsystem according to claim 1, wherein said abnormality-determining meansdetermines abnormality of said evaporative emission control system bycomparing a value of a parameter indicative a rate of change in thepressure within said fuel tank detected after said evaporative emissioncontrol system has been negatively pressurized by saidnegatively-pressurizing means with a predetermined reference value, saidpredetermined reference value being determined according to a timeperiod required for setting said evaporative emission control system tosaid predetermined negatively-pressurized condition by saidnegatively-pressurizing means.
 6. An evaporative fuel-processing systemaccording to claim 1, including means for purging evaporative fuelstored in said canister for a predetermined time period before theabnormality-determining process is started by saidabnormality-determining system.
 7. An evaporative fuel-processing systemaccording to claim 1, including fuel temperature-detecting means fordetecting the temperature of fuel contained in said fuel tank anddetermination-inhibiting means for inhibiting execution ofabnormality-determining process by said abnormality-determining systemwhen said fuel temperature detected exceeds a predetermined value.
 8. Anevaporative fuel-processing system for an internal combustion enginehaving an intake system, including an evaporative emission controlsystem having a fuel tank a canister containing an adsorbent, saidcanister having an air inlet port communicatable with the atmosphere, anevaporative fuel-guiding passage extending between said canister andsaid fuel tank, a first control valve arranged across said evaporativefuel-guiding passage, an evaporative fuel-purging passage extendingbetween said canister and said intake system and a second control valvearranged across said evaporative fuel-purging passage, said evaporativefuel-processing system having an abnormality-determining system whichcomprises: engine operating condition-detecting means for detectingoperating conditions of said engine; a third control valve for effectingand cutting off the communication of said air inlet port of saidcanister with the atmosphere; tank internal pressure-detecting means fordetecting pressure within said fuel tank; negatively-pressurizing meansfor setting said evaporative emission control system to a predeterminednegatively-pressurized condition by controlling said first to thirdcontrol valves when it is detected by said said engine operatingcondition-detecting means that said engine is in operation; a first rateof change-detecting means for detecting a rate of change in the pressurewithin said fuel tank caused by controlling opening and closing of saidfast control valve; a second rate of change-detecting means fordetecting a rate of change in the pressure within said fuel tank causedby closing said second control valve after said negatively-pressurizedcondition of said evaporative emission control system has beenestablished; and abnormality-determining means for determiningabnormality of said evaporative emission control system based on resultsof detection by said first and second rate of change-detecting means. 9.An evaporative fuel-processing system according to claim 8, includingtank condition-detecting means for detecting conditions of said fueltank wherein said abnormality-determining means carries out abnormalitydetermination when a predetermined time period has elapsed after saidevaporative emission control system was negatively pressurized saidpredetermined time period being corrected by a correcting time periodset in response to said conditions of said fuel tank detected by saidtank condition-detecting means.
 10. An evaporative fuel-processingsystem according to claim 8, wherein said abnormality-determining meansdetermines abnormality of said evaporative emission control system bycomparing a value of a parameter indicative of a rate of change in thepressure within the said fuel tank detected after said evaporativeemission control system has been negatively pressurized by saidnegatively-pressurizing means with a predetermined preference valueduring the negatively pressurizing, said predetermined reference valuebeing determined according to a time period required for setting saidevaporative emission control system to said predeterminednegatively-pressurized condition by said negatively-pressurizing means.11. An evaporative fuel-processing system according to claim 9, whereinsaid abnormality-determining means determines abnormality of saidevaporative emission control system by comparing a value of a parameterindicative of a rate of change in the pressure within the aid fuel tankdetected after said evaporative emission control system has beennegatively pressurized by said negatively-pressurizing means with apredetermined reference value during the negatively pressurizing, saidpredetermined reference value being determined according to a timeperiod required for setting said evaporative emission control system tosaid predetermined negatively-pressurized condition by saidnegatively-pressurizing means.
 12. An evaporative fuel-processing systemaccording to claim 8, wherein said abnormality-determining systemincludes fuel amount-detecting means for detecting an amount of fuelcontained in said fuel tank, said abnormality-determining meansdetermines the abnormality of said evaporative emission control systembased on results of detection by said first and second rate ofchange-detecting means and said fuel amount-detecting means.
 13. Anevaporative fuel-processing system according to claim 8, including meansfor purging evaporative fuel stored in said canister for a predeterminedtime period before the abnormality-determining process is started bysaid abnormality-determining system.
 14. An evaporative fuel-processingsystem according to claim 8, including fuel temperature-determiningmeans for detecting the temperature of fuel contained in said fuel tank,and determination-inhibiting means for inhibiting execution ofabnormality-determining process by said abnormality-determining systemwhen said fuel temperature detected exceeds a predetermined value. 15.An evaporative fuel-processing system for an internal combustion enginehaving an intake system, including an evaporative emission controlsystem having a fuel tank, a canister containing an adsorbent, saidcanister having an air inlet port communicatable with the atmosphere, anevaporative fuel-guiding passage extending between said canister andsaid fuel tank, a first control valve arranged across said evaporativefuel-guiding passage, an evaporative fuel-purging passage extendingbetween said canister and said intake system, and a second control valvearranged across said evaporative fuel-purging passage, said evaporativefuel-processing system having an abnormality-determining system whichcomprises: engine operating condition-detecting means for detectingoperating conditions of said engine, a third control valve for effectingand cutting off the communication of said air inlet port of saidcanister with the atmosphere, tank internal pressure-detecting means fordetecting pressure within said fuel tank; negatively-pressurizing meansfor setting said evaporative emission control system to a predeterminednegatively-pressurized condition by controlling said first to thirdcontrol valves when it is detected by said said engine operatingcondition-detecting means that said engine is in operation; andabnormality-determining means for effecting a determination as towhether or not said evaporative emission control system is abnormallyfunctioning, when a predetermined time period has elapsed during thenegatively-pressurizing by said negatively-pressurizing means.
 16. Anevaporative fuel-processing system according to claim 15, wherein saidabnormality-determining system includes evaporative fuel generationrate-detecting means for detecting a parameter of an amount ofevaporative fuel generated per unit time within said fuel tank, saidabnormality-determining means determining that said evaporative emissioncontrol system is abnormal on condition that said parameter indicativeof said amount of evaporative fuel generated per unit time within saidfuel tank is smaller than a predetermined value.
 17. An evaporativefuel-pressing system according to claim 15, including means for purgingevaporative fuel stored in said canister for a predetermined time periodbefore the abnormality-determining process is started by saidabnormality-determining system.
 18. An evaporative fuel-processingsystem according to claim 15, including fuel temperature-detecting meansfor detecting the temperature of fuel contained in said fuel tank, anddetermination-inhibiting means for inhibiting execution ofabnormality-determining process by said abnormality-determining systemwhen said fuel temperature detected exceeds a predetermined value. 19.An evaporative fuel-processing system for an internal combustion enginehaving an intake system, including an evaporative emission controlsystem having a fuel tank, a canister containing an adsorbent, saidcanister having an air inlet port communicatable with the atmosphere, anevaporative fuel-guiding passage extending between said canister andsaid fuel tank, an evaporative fuel-purging passage extending betweensaid canister and said intake system, and a purge control valve arrangedacross said evaporative fuel-purging passage, said evaporative emissioncontrol system comprising: a drain shut valve disposed to establish andshut off communication between said air inlet port of said canister andthe atmosphere; pressure-detecting means for detecting pressure withinsaid evaporative emission control system; negatively-pressurizing meansfor negatively pressurizing said evaporative emission control system;and abnormality-determining means for determining abnormality of saidevaporative emission control system based on an extent to which thepressure is maintained within said evaporative emission control system,said extent being detected based on the pressure within said evaporativeemission control system detected by said pressure-detecting means, aftersaid evaporative emission control system has been negatively pressuredby sad negatively-pressurizing means.
 20. An evaporative fuel processingsystem according to claim 19, wherein said abnormality-determining meansincludes pressure-holding means for holding the pressure within saidevaporative emission control system after said evaporative emissioncontrol system has been negatively pressurized by saidnegatively-pressurizing means, said abnormality-determining meansdetecting the extent to which the pressure is maintained within saidevaporative emission control system based on the pressure within saidevaporative emission control system detected by said pressure-detectingmeans, while the pressure within said evaporative emission controlsystem is held by said pressure-holding means.
 21. An evaporativefuel-processing system according to claim 20, wherein saidnegatively-pressurizing means opens said purge control valve and at thesame time closes said drain shut valve to negatively pressurize saidevaporative emission control system, and said pressure-holding meanscloses said purge control valve and at the same time closes said drainshut valve to hold the pressure within said evaporative emission controlvalve.
 22. An evaporative fuel-processing system according to claim 20,wherein said abnormality-determining means determines the extent towhich the pressure is maintained within said evaporative emissioncontrol means, by detecting a change in the pressure within saidevaporative emission control system detected by said pressure-detectingmeans over a predetermined time period, and determines that there is anabnormality in said evaporative emission control system, when thedetected change exceeds a predetermined value.
 23. In anabnormality-determining system of an evaporative fuel-processing systemof a vehicle for supplying and controlling an evaporative fuel adsorbedand held in a canister to an internal combustion engine in accordancewith an operating condition of the internal combustion engine,comprising an improvement wherein said abnormality determining systemincludes: first engine coolant temperature determining means fordetermining whether engine coolant temperature is lower than a firstpredetermined value at an initial start-up of said internal combustionengine; second engine coolant temperature determining means fordetermining when said engine coolant temperature is above a secondpredetermined value only if said first engine coolant temperaturedetermining means initially determines said engine coolant temperatureis lower than said first predetermined value; engine-operatingcondition-detecting means for detecting one or more predetermined engineoperating conditions and/or vehicle running conditions when said secondengine coolant temperature determining means determines said enginecoolant temperature is above said second predetermined value; andabnormality-determining means for determining an abnormality of saidevaporative fuel-processing system when said engine operating conditiondetecting means determines one or more predetermined engine operatingconditions and/or vehicle running conditions are satisfied.
 24. In anabnormality-determining system of an evaporative fuel-processing systemof a vehicle for supplying and controlling an evaporative fuel adsorbedand held in a canister to an internal combustion engine in accordancewith operating conditions of said internal combustion engine, comprisingan improvement wherein said abnormality determining system includes:abnormality determining means for determining an abnormality of saidevaporative fuel-processing system only when engine coolant temperatureis greater than a first predetermined value; and means for activatingsaid abnormality-determining means at engine start-up only when saidengine coolant temperature is less than a second predetermined valuewhich is less than said first predetermined value.
 25. Anabnormality-determining system for an evaporative fuel-processing systemof a vehicle, the evaporative fuel-processing system including a fueltank coupled to an intake passage of an internal combustion engine viaan evaporative fuel-purging passage, a canister disposed in line withsaid evaporative fuel-purging passage for adsorbing and holdingevaporative fuel from said fuel tank, said canister having an air inletport for introducing outside air into said evaporative fuel-processingsystem, and check valve means disposed between said fuel tank and saidcanister for maintaining a predetermined pressure in said fuel tank,said abnormality-determining system comprising: pressure-determiningmeans for determining pressure within said evaporative fuel processingsystem; first control valve means, arranged across an evaporative fuelguiding passage which is between said fuel tank and said canister, foropening and closing said evaporative fuel passage in response to a firstcontrol signal; second control valve means, arranged across saidevaporative fuel-purging passage between said canister and said intakepassage, for opening and closing the evaporative fuel-purging passage inresponse to a second control signal; a third control valve means foreffecting and cutting off communication with said air inlet port of saidcanister with the atmosphere in response to a third control signal; andan abnormality-determining means for determining presence or absence ofevaporative fuel leakage in said evaporative fuel-processing system whenone or more predetermined engine operating conditions and/or vehiclerunning conditions are satisfied, said abnormality determining meansselectively generating said first, second and third control signals todetermined presence or absence of evaporative fuel leakage.
 26. Anabnormality determining system according to claim 23, wherein saidsecond predetermined value is greater than said first predeterminedvalue.
 27. An abnormality determining system according to claim 23,wherein said one or more conditions comprise a vehicle velocity state oran engine rotational speed state.
 28. An abnormality determining systemaccording to claim 27, wherein said one or more conditions furthercomprise one of a vehicle velocity fluctuation over time, intake pipepressure, and throttle opening degree.
 29. An abnormality determiningsystem according to claim 25, wherein said one or more conditionsinclude an engine coolant temperature being greater than a predeterminedvalue.
 30. An abnormality determining system according to claim 29,wherein said one or more conditions comprise a vehicle velocity state oran engine rotational speed state.
 31. An abnormality determining systemaccording to claim 30, wherein said one or more conditions furthercomprise one of vehicle velocity fluctuation over time, intake pipepressure, and throttle opening degree.
 32. An abnormality determiningsystem according to claim 24, wherein said abnormality-determining meansincludes engine-operating condition-detecting means for detecting one ormore predetermined engine operating conditions and/or vehicle runningconditions before executing said abnormality-determining means, said oneor more predetermined engine operating conditions and/or vehicle runningconditions being from a group including a vehicle velocity state, anengine rotational speed state, a vehicle velocity fluctuation over time,an engine rotational speed, and a throttle opening degree.
 33. Anabnormality determining system for detecting an abnormality in anevaporative fuel processing system having a fuel tank storing an amountof fuel, an evaporative fuel-guiding passage extending between said fueltank and a canister, an evaporative fuel-purging passage through whichfuel vapor stored in said canister is purged into an intake passage ofan internal combustion engine and a purge control valve arranged acrosssaid evaporative fuel-purging passage to allow a purge operation byopening of said purge control valve, said abnormality-determining systemcomprising: negatively-pressurizing means for introducing a negativepressure from said intake passage of said internal combustion engineinto said evaporative fuel processing system; pressure-detecting meansfor detecting pressure within said evaporative fuel processing systemwhen negative pressure is introduce therein by saidnegatively-pressurizing means; abnormality-determining means fordetermining an abnormality in said evaporative fuel processing systembased upon pressure in said evaporative fuel processing system, saiddetermination using values supplied by said pressure-detecting means;and negative pressure controlling means for controlling saidnegatively-pressurizing means so as to prohibit negatively-pressurizingof said negatively-pressurizing means while said abnormality-determiningmeans is determining the abnormality, when said negative pressure isintroduced into said evaporative fuel processing system by saidnegatively-pressurizing means wherein suctioning of said fuel vaporcollected in the fuel tank with air into the engine results influctuation of an air-fuel ratio.
 34. An abnormality-determining systemaccording to claim 33 , further comprising a control valve for effectingand cutting off communication of an air inlet port of said canister withthe atmosphere wherein; said negatively-pressurizing means comprisescontrolling means for controlling said purge control valve and controlvalve, negative pressure inside said intake passage being introducedinto said evaporative fuel processing system by closing said controlvalve and opening said purge control valve.
 35. An abnormalitydetermining system for detecting an abnormality in an evaporative fuelprocessing system having a fuel tank storing an amount of fuel, anevaporative fuel-guiding passage extending between said fuel tank and acanister, an evaporative fuel-purging passage through which fuel vaporstored in said canister is purged into an intake passage of an internalcombustion engine and a purge control valve arranged across saidevaporative fuel-purging passage to allow a purge operation by openingof said purge control valve, said abnormality-determining systemcomprising: negatively-pressurizing means for introducing a negativepressure from said intake passage of said internal combustion engineinto said evaporative fuel processing system; pressure-detecting meansfor detecting pressure within said evaporative fuel processing systemwhen negative pressure is introduce therein by saidnegatively-pressurizing means; negative pressure controlling means forcontrolling said negatively-pressurizing when said negative pressure isintroduced into said evaporative fuel processing system by saidnegatively-pressurizing means wherein suctioning of said fuel vaporcollected in the fuel tank with air into the engine results influctuation of an air-fuel ratio; and abnormality-determining means fordetermining an abnormality in said evaporative fuel processing systembased upon pressure in said evaporative fuel processing system, saiddetermination using values supplied by said pressure-detecting means,said system further comprising means for determining a fuel amountstored in said canister and said negatively-pressurizing means isactivated based upon said fuel amount stored in said canister.
 36. Anabnormality determining system according to claim 35, said fuel amountstored in said canister is determined by time elapsed since said purgecontrol valve was opened.
 37. An abnormality determining systemaccording to claim 36, wherein said opening and closing of said purgecontrol valve is controlled to be linearly changed.
 38. An abnormalitydetermining system according to claim 35, wherein said abnormalitydetermining means determines the existence or non-existence of amalfunction of said evaporative fuel processing system by comparing arate of pressure change inside said evaporative fuel processing systemover a predetermined period of time with a predetermined value, saidrate of pressure change being obtained by using pressure values detectedand supplied by said pressure detecting means.